This invention relates to a space diversity receiver apparatus and, more particularly, to a space diversity receiver apparatus for receiving signals by two spatially separated antennas, controlling the phase of the signal received by one of the antennas, and combining the phase-controlled signal and the signal received by the other antenna, thereby reducing the effects of fading produced by a wireless communication medium.
The frequency of occurrence, depth and time of occurrence of fading which occurs in a wireless communication medium differ depending upon the location at which the receiving antenna is installed. In space diversity reception, this property is utilized by placing two antennas at positions having little fading correlation and either combining the signals received by the two antennas or switching between the two signals to reduce fading.
FIG. 20 is a diagram showing the construction of a conventional space diversity receiver apparatus of the type which applies a minimum amplitude deviation method. The apparatus includes intermediate frequency output units 10a, 10b which output intermediate frequency signals S1, S2, respectively, obtained from reception signals of first and second antennas (not shown) by high-frequency amplification, frequency conversion and intermediate frequency amplification, a phase control circuit 11 for controlling the phase of the intermediate frequency signal S2, a combiner 12 for combining the intermediate frequency signal S1 and an output signal S2' of the phase control circuit 11, an AGC circuit 13 for compressing output level, an analog detector 14 for detecting the signal levels of three frequencies (center frequency f.sub.0, high-side frequency f.sub.0 +.DELTA.f and low-side frequency f.sub. -.DELTA.f) in the intermediate frequency band, an AD converter 15 for converting the signal levels of these frequencies to digital values, and a controller 16 of microcontroller construction for exercising control so as to flatten the frequency characteristic in the intermediate frequency band. There are cases where the frequency characteristic in the intermediate frequency band is flat (FIG. 21A), increasing (FIG. 21B) or decreasing (FIG. 21C). The controller 16 performs phase control in such a manner that the frequency characteristics of FIGS. 21B and 21C will become the frequency characteristic shown in FIG. 21A.
The detector 14 has first through third detectors 14a-14c for detecting and outputting the signal levels of the three frequencies, namely the low-side frequency f.sub.0 -.DELTA.f, center frequency f.sub.0 and high-side frequency f.sub.0 +.DELTA.f, respectively. The AD converter 15 has first through third AD converters 15a-15c for converting the outputs of the first through detectors 14a-14c, respectively, to digital signals.
Each of the detectors 14a-14c has the construction shown in FIG. 22. Specifically, a bandpass filter BPF passes one of the frequency components f.sub.0 -.DELTA.f (low side), f.sub.0 (center) and f.sub.0 +.DELTA.f (high side). The bandpass filter BPF is constituted by an already existing printed-board filter or by such elements as a coil, capacitor and resistor mounted on a printed circuit board. A rectifier circuit RCF, which rectifies the output signal of the bandpass filter BPF, is obtained by mounting diodes or the like on a printed circuit board. A low-pass filter LPF averages the output signal of the rectifier circuit RCF and outputs the average. An operational amplifier OPA amplifies the output of the low-pass filter LPF and outputs the detected voltage. Thus, the bandpass filter BPF extracts and outputs the desired frequency component from the intermediate frequency signal, the rectifier circuit RCF rectifies the desired frequency component extracted by the bandpass filter BPF, the low-pass filter LPF averages and delivers the output signal of the rectifier circuit RCF, and the operational amplifier OPA amplifies the average voltage signal and outputs the detected voltage.
In the space diversity receiver apparatus of FIG. 20, the phase control circuit 11 controls the phase of the received signal S2. The combiner 12 combines like phases of the received signal S1 and phase-controlled signal S2' and outputs a signal S3 via the AGC circuit 13. The detector 14 detects and outputs the signal levels of the three frequencies (center frequency f.sub.0, high-side frequency f.sub.0 +.DELTA.f and low-side frequency f.sub.0 -.DELTA.f) in the intermediate frequency band, and the AD converter 15 converts the signal levels of these frequencies to digital values and outputs the digital values. The controller 16 controls the phase of the signal S2 in such a manner that an amplitude deviation among these frequencies is eliminated. Repeating the operation described above flattens the frequency characteristic in the intermediate frequency band. That is, by virtue of the above-described operation, the phases of the interference components of the received signals S1, S2 are mutually shifted by .pi. and the signals are then combined, thereby canceling out the interference components so that the signal S3, which influenced little by interference, can be obtained as an output.
FIG. 23 is a diagram useful in describing the minimum amplitude deviation method. The received signals S1 and S2 are the results of combining direct waves (indicated by the fine solid lines) and interference waves (the dashed lines) caused by fading. With the minimum amplitude deviation method, the phase of the received signal S2 is controlled so that the phase difference between the interference wave S2b of the received signal S2 and the interference wave S1b of the received signal S1 becomes .pi., after which the two signals are combined and delivered as an output. In accordance with the minimum amplitude deviation method, interference components are canceled to make it possible to output the signal S3, which exhibits little interference.
With this space diversity receiver according to the prior art, the detector basically is a circuit on which analog components are mounted. A problem which arises is the space required for the large mounting area.
Further, since the conventional detector is of analog construction, elements exhibit variance in terms of pass loss in the bandpass filter, the rectifying characteristic of the rectifier circuit and pass loss in the low-pass filter. The problem which arises is that the output value of the detected voltage must be standardized. Standardization means applying a correction in such a manner that the detected voltage output will take on an already set value when an intermediate frequency input signal has a certain fixed value.
An additional problem is that the conventional analog detector must be provided with compensating circuits to compensate for temperature and fluctuations in power supply.
Further, in a space diversity receiver of the type which applies the minimum amplitude deviation method, the detection time of the detector is strongly related to control speed. In the conventional analog detector, the delay time possessed by the rectifier circuit is large and it is necessary to convert a plurality of frequency components from analog to digital quantities. Consequently, detection time cannot be shortened and faster phase control cannot be achieved.
Further, a space diversity receiver of the type which applies the minimum amplitude deviation method is not effective if the difference between the reception levels of the two antennas is too large. Conventionally, phase control based upon the minimum amplitude deviation method is performed even if this is the case. This means that when one of the signal levels recovers to thereby reduce the level difference between the received signals from the two antennas, the combined signal level cannot be enlarged immediately.
Further, with the space diversity receiver of the type which applies the minimum amplitude deviation method, opposite phases of the interference components of the two received waves are combined. As a result, a deviation in amplitude can be reduced. However, depending upon the phase relationship between the direct wave and interference wave, combined power also declines when the amplitude deviation is minimized. This degrades the C/N ratio. In addition, code error rate is degraded by thermal noise.
Further, with the space diversity receiver of the type which applies the minimum amplitude deviation method, accurate phase control cannot be carried out owing to a variance in the amplitude deviation characteristic of the analog circuitry provided in the path of the antenna output.